1. Field of the Invention
The invention relates to diazotypes and more particularly relates to erasable diazotype reproduction materials and the method of their preparation.
2. Brief Description of the Prior Art
Diazotypy is a process for making copies from translucent originals by exposing them in contact with a diazotype copying sheet to ultraviolet light and by developing the diazotype sheet thereafter with ammonia or a liquid developer. The diazotype process because of its ease and economics has found wide application during the last 50 years and its principles are well known; see "Light-Sensitive Systems" by Jaromir Kosar, John Wiley & Sons, Inc., New York (1965).
Diazotype copying materials are made by applying a light-sensitive diazo coating to the surface of a base, such as foils, opaque paper, translucent paper, or cloth. The active components of the light-sensitive coating are monomolecular, crystalline, not self-supporting and need a resinous or a fibrous substance as a support.
If paper is used as the base support, the diazotype coating penetrates into and disperses in the top layer of the paper and the diazotype print lines of the final copy are well anchored within the fibrous structure of the paper.
Diazo copies in the engineering field are often used for further design work, and it is desirable that corrections of print lines be made easily and not cut noticeably into the paper, thus damaging it for further markings. Corrections have been made in the past through the use of chemical eradicator solutions which destroy the print line through reduction or oxidation of the print dye. However, such chemicals also attack the paper base to a certain degree and leave a cockled and discolored spot that often shows up on reprinting as an undesired marking. Corrections on diazotypes are often required on intermediate diazotype prints, also called "reproducibles" or "second originals". These are diazotypes coated on a translccent base and the print lines are composed of one or more dyes which are opaque to ultraviolet light.
Erasable diazotype materials and their manufacture have been disclosed and described in U.S. Pat. Nos. 3,923,518 and 4,058,399. Diazotype materials prepared with these processes are easily erasable, but prints are often not resistant to Scotch tape which, when removed, pulls off all or a substantial part of the coating layer. Prints are also very sensitive to scratching. Even light scratching of the print surface with a finger nail removes the entire coating in the contact zone. Such coatings also exhibit a certain grainness which affects print line continuity and makes them substantially inferior to film coatings for reprinting applications.
Erasable diazotype materials of the prior art can be considered as a base material with laminated layers which are prepared by applying to a mostly transparent, paper base, a barrier coating which is impervious to the components of a single or double oeercoating with pigments, resins and diazotype materials. The barrier coatings excluded polyvinylacetates which exhibit some affinity to diazotype components. After image exposure, such materials are developed (mostly with hot ammonia) in conventional diazotype copying machines. Developed copies can be erased for necessary corrections. Handling of the material during the copying steps and afterwards usage for making reprints requires mechanical stability of the material so that no valuable information on the print is deleted.
The above-described prior art processes depend on a precarious balance of the composition of the coating layers.
The barrier layer resins have no affinity to the overcoating which adheres to the barrier layer by purely adhesive forces. The reprographic element of these materials is contained in the overcoatings with a resin system that has an adequate affinity to the azodyes of the developed prints, the affinity being increased by (mainly) silica pigments.
An inherent problem of such coating systems is the precarious balance between resin and pigment concentration that finally control erasability, rate of development, print contrast, ease of processing and mechanical print surface resistance.
Higher resin concentrations improve mechanical resistance, but affect negatively erasability, rate of development and passage through hot developing equipment.
Higher pigment concentrations improve rate of development, print contrast (up to a certain limit) and ease of erasability, but decresse mechanical print surface resistance.
Moreover, coating defects such as streaks or orange peel effects occur easily due to the necessary relatively heavy coating weight.
The azo print dye is not truly "dissolved" in the pigment-resin matrix of the overcoating, but the dye particles are rather dispersed and generate a certain degree of print dye graininess which reproduces on reprints in the form of lesser print line continuity.
The present invention overcomes all of the above shortcomings and provides diazotype second originals on translucent paper, which excel by a fine grain reproduction with high reprint contrast and which process easily through conventional printing and developing equipment. They can be easily erased, but are resistant to scratching, shop handling, rubbing and Scotch tape removal.